1. Field of the Invention
The present invention relates to analog signal filters. More particularly, the invention concerns a signal filter providing an analog impedance whose value is selectively established by digital control signals and may be changed over time in response to feedback, timing, or other desired input.
2. Description of the Related Art
Analog filters are used in a variety of applications, such as reducing noise, dampening output signals, reducing ringing, decreasing signals of selected frequencies, amplifying selected frequencies, and converting digital signals to analog.
Analog filters include discrete circuit components such as resistors, capacitors, inductors, and the like. Active filters additionally include amplifiers such as transistors, operational amplifiers, differential amplifiers, and the like. One of the most basic analog filters is the so-called xe2x80x9cR-Cxe2x80x9d filter, in which a resistor and capacitor are coupled in series, and one or the other is coupled to electrical ground. The R-C filter""s electrical output, taken between the resistor and capacitor, varies depending upon the values chosen for the resistor (R) and the capacitor (C). For instance, if the filter is constructed with a greater R-C product, the filter tends to have a smaller bandwidth and less responsively follow its input signal.
Designers of analog filters, such as R-C filters, seek to minimize signal acquisition time, tracking errors, and power consumption. When a given signal is input to an analog filter, the filter begins to change its output to reach a given steady-state or xe2x80x9ctargetxe2x80x9d output, the ultimate characteristics of which depend upon the values and interconnections of the filter""s circuit components. Signal acquisition time refers to the time in which the filter generates the target output signal within some tolerance. Thus, signal acquisition time concerns the filter""s ability to provide an output signal that quickly responds when its input signal changes or when such a filter is initially presented with an input signal. Tracking error refers to the ongoing error between the filter""s actual output signal and the target output signal. Thus, tracking error concerns the filter""s ability, after initially responding to an input signal change, to provide an output signal that accurately tracks the target output signal. Bandwidth concerns the signal filter""s operable range of input signal frequencies.
Unfortunately, the interests of maximum bandwidth with minimal signal acquisition time, minimal tracking error, and low power consumption tend to oppose each other. Broader bandwidth is compatible with faster signal acquisition, but comes with the cost of poor signal tracking when the input signal is noisy and also at the expense of higher power consumption. Conversely, accurate signal tracking means poorer signal acquisition and narrower bandwidth. Therefore, designers often sacrifice one or more less important filter attributes in favor of other, more important ones. Depending upon the particular application in which a signal filter is utilized, these sacrifices can have tangible effects. In a household thermostat, for example, longer acquisition times in a room thermostat mean longer wait times until a desired room temperature is achieved. In that same household thermostat, a poor tracking error may result in room temperature that varies by five or ten degrees from the desired room temperature.
As mentioned above, circuit designers are unable to satisfy the competing goals of maximum bandwidth, minimal signal acquisition time, minimal tracking error, and low power consumption. This forces circuit designers to design filters that necessarily sacrifice one or more of these properties. Known analog signal filters, then, are not completely adequate in all respects due to certain unsolved problems.
Broadly, the present invention concerns a signal filter providing an analog impedance whose value is established by digital control signals and may be changed over time in response to feedback signals, timing signals, or other desired input. In one embodiment, the signal filter is implemented to provide a first-order R-C filter and includes a resistance assembly and a reactance assembly. The resistance assembly has multiple parallel signal paths sharing a common input and a common output. Each signal path exhibits a prescribed electrical resistance, such as a resistor or inherent line resistance, and includes a switch to selectively disable or enable the resistance. Between the common input and output, the signal paths provide a collective resistance which varies depending upon which switches have been activated.
The reactance assembly includes at least one reactance element, such as a capacitor or inductor, coupled between the resistance assembly""s common output and electrical ground. If adjustable reactance is desired, the reactance assembly may be constructed to include multiple parallel signal paths coupled between the output and electrical ground, where each signal path in the reactance assembly includes an electrical reactance and with a switch to selectively disable or enable the reactance. Thus, the signal paths of the reactance assembly provide a collective reactance between the output and electrical ground.
A digital controller is coupled to the switches. The controller is programmed, constructed, configured, or otherwise built to adjust the resistance assembly""s collective resistance (and reactance assembly""s collective reactance, if applicable) by selectively activating the switches. The controller circuit may act in response to a feedback signal (such as the signal at the common output), a clock signal, a custom logic signal, or any other desired signal.
The foregoing features may be implemented in a number of different forms. For example, the invention may be implemented to provide a method of signal filtering utilizing adjustable filter parameters. In another embodiment, the invention may be implemented to provide an apparatus such as a configurable signal filter. In still another embodiment, the invention may be implemented to provide a signal-bearing medium tangibly embodying a program of machine-readable instructions executable by a digital data processing apparatus to control an adjustable signal filter to operate as shown herein. Another embodiment concerns logic circuitry having multiple interconnected electrically conductive elements configured to operate an adjustable signal filter to operate as discussed above.
The invention affords its users with a number of distinct advantages. For example, the present invention""s signal filter can be initially configured to provide fast acquisition time and then re-configured to provide minimize tracking error. This is possible by operating digital control circuitry to change the filter""s analog impedance over time according to feedback, timing, or other desired signals. The adjustable filter of this invention also offers the advantage of low power consumption because it permits dynamic increases in resistance during periods when certain sacrifices in performance are tolerated.
As another advantage, the invention offers an embodiment where all signal paths and their resistors are implemented in an integrated circuit. This conserves valuable integrated circuit xe2x80x9cpads,xe2x80x9d because the individual pads are not needed to separately interface the digital controller with each different signal path""s resistor. The invention also provides a number of other advantages and benefits, which should be apparent from the following description of the invention.